During the publishing of the draft, the status of the current version is changed to Archived. However, the current version is retrieved from the object which is cached in a global variable (https://github.com/ezsystems/ezpublish-legacy-ee/blob/master/kernel/content/ezcontentoperationcollection.php#L134), so the current version might be outdated if another version is published simultaneously. If this happens, the Content Object will have two Published versions.